Heart Rate Zone 3 Aerobic or grey Zone?

Zone 3 is indeed aerobic, because the body still uses quite a lot of oxygen to power itself. However, if it's done too often (as you say running most of the time) or done as a long interval then it adds a lot of fatigue. So, in the end the return from such training is not that good, which is why a popular advice is to use a polarized model to be on the safe side. Training in Zone 3 has a lot of benefits if done right - after all, for most amateurs that correlates a lot with marathon pace. There's a good article about heart rate training zones where the difference between them is explained

You're really asking more of a general training or coaching question, not a Garmin Connect Web question. That said, the amount of training you do in Zone 3 should depend on your goals and volume. Endurance athletes who do a high volume of training usually get the best results by spending about 80% of the time in Zones 1-2 and only 20% in Zones 3-5. But those aren't fixed numbers. Ask a qualified coach to see what would work for you. 

Hi ir77

I think there is a lot of confusion around this as different plans use different % for the zones and we as runners only hear zone 2 zone 2.

Well if you look at the mafetone method which says all runs should be easy and in there "zone 2" it will match up closely with zone 3 in Garmin. If you read Garmin definition of zone 3 it says "easy runs should be at this zone" . These zone definitions are based on MHR. Zone 2 MHR in other program sits at 60-75% which also matches Garmin's zone 3.

Garmin also says If you modify the zones for another program there definitions may no longer match.

Most runners on these forums switch to HRR which interestingly for myself (and likely most ) makes zone 2 look closer to zone 3 and matches more closely with the zone 2 training programs ..

Important to note nothing really has changed physically.

To Garmin's defaults zone 3 is easy - if you change only from MHR to HRR zone 2 is easy. Whatever makes more sense to do - do that whether you call it zone 3 or zone 2 - just run easy conversationally.  

My personal tip is when you're fresh do for an easy run conversationally (nose breathing ) - speed up until this isn't the case and then back off a bit. DONT LOOK at your watch on this run . Go for 40 min or so at this pace and when you get home set your pace accordingly . You'll likely notice when set to MHR with default zones that matches zone 3 and when set to HRR with the default zones that's zone 2.